Measuring the beam landing characteristic of a shadow-mask cathode-ray tube

ABSTRACT

A method of adjusting the electron beam landing in a color display tube on the screen of which phosphor dots are provided which are impinged upon by the electron beams so that they luminesce with different colors. One electron beam is activated while the picture to be displayed is a white raster. Alternating current and direct current fields are adjusted in such a manner that the spots of the electron beams are arranged symmetrically relative to the phosphor dots, which may be found with the aid of a detector arrangement, for example, a photomultiplier which is exclusively sensitive to the color corresponding to the active electron beam. The landing characteristic may be measured and the color purity may be adjusted.

United States Patent Oxenham 1 1 Mar. 27, 1973 [54] MEASURING THE BEAM LANDING 2,657,331 10 1953 Parker ..315/26 CHARACTERISTIC ()F A SHADOW. 2,604,534 7/1952 Graham. .....l78/6.8 2,851,525 9/1958 Kihn 1 ..178/7.7 MASK CATHODE RAY TUBE 2,630,548 3/1953 Muller ..315/21 [75] Inventor: John Kenneth Oxenham, Blackburn,

England Primary ExaminerCarl D. Quarforth Assistant Examiner-P. A. Nelson [73] Assignee. Philips Corporation, New York, Atwmey Frank Trifafl [22] Filed: July 16, 1970 [57] ABSTRACT [21] Appl. No.: 55,441 A method of adjusting the electron beam landing in a color display tube on" the screen of which phosphor dots are provided which are impinged upon by the [30] Fol-mg Appllcat'on Pnonty Data electron beams so that they luminesce with different Aug. 5, 1969 Great Britain ..39,l56/69 r n electron am is activated while the picture to be displayed is a white raster. Alternating cur- [521 (LS C1 ..315/13 C, 315/13 CG, 315/21 C, rent and direct current fields are adjusted in such 21 5 2 CD manner that the spots of the electron beams are ar- 51 1111. C1 ..H0lj 29/54 ranged symmetrically relative to the Phosphor 401$, 58 Field of Search ..315/13, 13 c, 13 CD, 21 c, which may be found with the aid of a detector [56] References Cited UNITED STATES PATENTS 2,611,816 9/1952 Darke ..178/5.2

rangement, for example, a photomultiplier which is exclusively sensitive to the color corresponding to the active electron beam. The landing characteristic may be measured and the color purity may be adjusted.

11 Claims, 5 Drawing Figures Patented March 27, 1973 3,723,801

3 Sheets-Sheet 2 0 I V W i 15 19 1 Q A.C. SOURCE r J D.C.SOURCE) A.C. SOURCE L 4 I 0.c. SOURCE lZ r I 25 2sI 27 2a I l I (5 l II I I 30 DIFFERENTIAL] AMPLIFIER DIFFERENTIAL I AMPLIFIER i n 24 23 33 COLOR I FILTER PHOTOMULTIPLIER INVENTQK JOHN K. OXENHAM Patented March 27, 1973 3 Sheets-Sheet 5 INVENTOR.

JOHN K. OXENHAM AGEN\ MEASURING THE BEAM LANDING CHARACTERISTIC OF A SHADOW-MASK CATIIODE-RAY TUBE The invention relates to a method of adjusting the electron beam landing in a color picture display tube on the screen of which phosphor dots of luminescent material are provided which are impinged upon by at least two electron beams so that they luminesce with different colors at least part of the screen being scanned by the electron beam due to the action of deflection coils, only one electron beam being activated during said adjustment, while the picture to be displayed is without picture content.

It is well-knowri that the operation of a shadow-rnask color display cathode ray tube requires that the electrons from three electron guns provided in the tube pass through holes in the shadow-mask and strike red, green or blue color phosphor dots provided on the tube screen respectively. This is normally accomplished by depositing the phosphor dots photographically using the shadow-mask as a negative and placing the light source used for exposure approximately in the position of the corresponding electron beam as it would pass through deflector coils placed around the neck of the tube. In the subsequent operation of the tube, the electron beams are made to follow approximately the same paths as did the light rays used for the exposure.

Due to tolerances of tube manufacture, the electron beams may not pass exactly through the assumed deflection centers, and so magnetic correction fields are applied to the three beams simultaneously so that they do indeed pass through these deflection centers. This correction is known as the purity correction and may consist either of two variable perpendicular fields, or two fields of constant magnitude which can be rotated. The former system is used for convenience in factory test boards, and the latter system is used in commercial television receivers. Further purity adjustment is made by moving the deflector coils along the neck of the tube so that their deflection plane corresponds with the deflection plane which was assumed during exposure.

Despite the various corrections that are included when the phosphor dots are laid down, it is not possible to make the electron spot formed by the incidence of each one of the electron beams on the tube screen substantially concentric with the corresponding phosphor dots all over the screen, due to geometrical and electron-optical considerations. The position of the electron spot relative to the corresponding phosphor dots, i.e. the beam landing characteristic, will vary all over the screen. A known measuring method and a method of adjusting the purity is the so-called microscope method described, for example, in the publication Philips Product Note No. 5: Color purity adjustment".

In this method the picture to be displayed without picturecontent is a so-called white raster and the beam landing pattern is observed with a microscope. This has several disadvantages:

a. It is a time-consuming operation because the screen of each tube may have to be measured in many positions while the color purity adjusting members are present on the rear side of the screen;

b. A clear image is not formed in the microscope because of the thickness of the tube face plate, and

because the outside surface of the face plate is not normally optically perfect;

c. It is difficult to ascertain the center of each phosphor dot because it may have a ragged edge, and unless the measurements are taken over a number of dots in a given area, a true average landing is not obtained. This makes the job even more time-consuming and the accuracy of measurement is, at best, not very high.

d. In tubes in which the incoming electron beam is thicker than the phosphor dot, which is either or not surrounded by an absorbing material, this method cannot be used in a simple manner; such a picture tube is described in U S Pat. No. 3,146,368.

It is an object of the invention to provide a method of measuring the beam landing characteristic accurately during the manufacture of the picture tubes and of correcting mislanding in finished, tubes, i.e. the color purity adjustment. To this end the method according to the invention is characterized in that a detector arrangement is placed in front of the screen which detector is substantially insensitive to the colors with which the phosphor dots luminesce which do not correspond to the electron beam activated, a first magnetic field being generated which causes the displacement of the electrons into a first direction, two different values successively being given to the field in such a manner that the distance between the resultant spots of the electron beam on the screen is of the same order as the largest of the diameters of a phosphor dot or the spot of the electron beam on the screen, a first direct current field of adjustable intensity being generated which causes the displacement of the electrons into the first direction and the intensity of the direct current field being adjusted in such a manner that the detector arrangement indicates substantially equal values for the two different values of the field.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings in which;

FIG. 1 shows various positions of an electron spot relative to a phosphor dot obtained by the application of a stepped magnetic field for purity correction to a shadow-mask color display cathode ray tube; said phosphor dot being thicker than the electron beam;

FIG. 2 shows how orthogonal magnetic fields for purity correction can be varied as a function of time in order to provide the spot positions of FIG. 1; and

FIG. 3 shows apparatus generating these fields,

FIG. 4 in a modification of a system of coils forming part of the apparatus of FIG. 3.

FIG. 5 corresponds to FIG. 1 if the incoming electron beam is thicker than the phosphor dot.

In FIG. 1 a phosphor dot 1 which forms part of the display screen of a shadow-mask color display cathode ray tube is made of a particular luminescent material which emits one of the colors red, green or blue when excited by electrons. In operation the electrons are directed at the dot from an electron gun provided in the tube and reach the dot through a hole in a shadowmask, which is provided in the usual way to prevent electrons from the other two guns of the tube from reaching dots which emit this color. In practice the electrons from the corresponding gun are not likely to impinge on the center of the dot but rather to impinge on an area 2 in FIG. 1. This mislanding is normally corrected in the color television receiver by applying a pair of magnetic fields in the X and Y directions in order to move the electron spot 2 to a position 3. The mislanding may be different for different parts of the screen area, and it is necessary during tube manufacture to measure the landing characteristic thus defined and in addition it is necessary to correct the mislanding in finished tubes.

Referring to FIGS. 2a and 2b, if magnetic fields for purity correction in the X and Y directions are applied to the tube in a stepped manner as shown, where vertical distance in FIGS. 2a and 2b represents the resulting shift in the electron spot in the two directions respectively, it will be seen that the simultaneous application of these correction fields will result in successive movement of the spot 2 to positions 4, 5,6 and 7 (FIG. 1). It will be seen that, regardless of the DC level superimposed on the two magnetic fields the positions 4, 5, 6 and 7 will always be the same relative to one another but that variation of the DC. levels will result in each of these four positions moving in the same way relative to the center of the dot 1. Suitable choice of the DC components of these fields can be made to move the four positions so that they are symmetrical about the center of the dot 1. The required D.C. levels to achieve this with the mislanding shown in FIG. 1 are shown in broken lines in FIG. 2. It will be seen that, if the four positions are symmetrical about the center of the dot 1 then a detector for the resulting radiation from the dot 1 will give the same light intensity when the electron spot is in each of these four positions, and thus when such equal light intensities are obtained the DC. levels of the two purity correction fields are an indication of the initial mislanding of the spot 2 on the dot 1. It will be obvious that the distances between the centers of the positions 4 and 6 and between and 7 must be in the order of the diameter of spot 2 or of the diameter of dot 1, whichever is greater, if the spot and dot are to overlap in the desired manner for each position. More exactly, this condition is: said distances must be smaller than the sum of the dot diameter and the spot diameter, and it must be larger than the difference between the two diameters. This condition applies when in FIG. 1 the diameter of positions 4 and 6 and 5 and 6, respectively, are in a straight line with that of phosphor dot 1. Otherwise the two aforementioned extreme values are reduced.

FIG. 3 shows apparatus for carrying out a method according to the invention. In FIG. 3 two magnetic fields are applied to the tube in the X and Y directions by means of electromagnet coils 8 and 9, respectively, provided on the neck of the display tube. These coils are supplied with variable D.C. by'means of current sources 10 and 11, respectively, via two meters 34 and 12. An A.C. current is superimposed on the direct current from the supply source 10 by means of a positive current supply source 13 and a negative current supply source 14. The sources 13 and 14 are connected to the electromagnet coil 8 by means of the switching arrangement 15 shown (in practice this switch will normally be an electronic switch). In a similar manner an alternating current is superimposed on the direct current from the supply source 11 by means of a positive current supply source 16 and a negative current supply source 17 which are connected to the electromagnet coil 9 by means of a switch 18 which is ganged to the switch 15. When the switches 15 and 18 are rotated the resulting current through the coils 8 and 9 will be similar to those shown in FIGS. 2a and 2b, respectively, and thus give a relative movement of the spot formed by the electron beam on the tube screen to the four positions 4, 5 6 and 7. The positions of switch 15 which give each of these spot positions are denoted by 19, 20, 21 and 22, respectively. The D.C. levels of the fields produced by the coils 8 and 9, i.e. the zero levels in FIGS. 2a and 2b, are varied simply by varying the outputs of the supply sources 10 and 11, respectively.

The app aratusof FIG. 3 also includes a photo-multiplier 23 for detecting the radiation from the spot 1 when it is excited by electrons. The anode of this photo-multiplier is connected to the master contact of a further switch 24 which is ganged to the two switches 15 and 18. Each of the contact positions of the switch 24 is connected to resistors 25, 26, 27 or 28 each one of which constitutes the load for the photo-multiplier 23. The inputs of a differential amplifier 29 are connected to the resistors 25 and 27, respectively, while the inputs of a further differential amplifier 30 are connected to the resistors 26 and 28, respectively. These differential amplifiers feed output meters 31 and 32 respectively.

The beam landing characteristic of the shadow-mask cathode ray tube is measured utilizing the apparatus of FIG. 3 as follows:

The purity coils 8 and 9 are positioned on the neck of the tube which is supplied with its normal operating potentials except that only one of its three guns is switched on, while the picture to be displayed is a white raster. The photo-multiplier 23 is positioned to view part of the tube screen area through a filter 33 which transmits light resulting from excitation of the phosphor dots on the tube screen which correspond to the energized gun and is opaque to radiation which would be emitted by the other phosphor dots if they were excited. The filter 33 may be omitted if the photomultiplier 23 is sensitive to light resulting from excitation of the phosphor dots corresponding to the energized gun and insensitive to radiation which would be emitted by the other phosphor dots if they were excited. The electron beam emitted by the energized gun is caused to scan at least the part of the tube screen which is viewed by the photo-multiplier 23 and in practice it scans the complete tube screen by means of the normal scanning coils. I

Before the current supply sources 13, 14, 16 and 17 are energized the output of the current sources 10 and 11 are slowly increased from zero, if necessary, until an output is obtained from photo-multiplier 23. Thus approximate alignment of the electron beam from the energized gun on the corresponding dots is obtained. The current supply sources 13, 14, 16 and 17 are now energized and the switches 15, 18, 24 are rotated. DC. current supply source 10 is then varied until zero output is obtained from differential amplifier 29, thus giving accurate alignment of the electron beam with the phosphor dots in the X direction. Current supply source 11 is likewise varied until zero output is obtained from differential amplifier 30, this giving accurate alignment of the electron beam with the phosphor dots in the Y direction. Since the outputs of photo-multiplier 23 for the positions 19, 21, and 22, 20, respectively, are not simultaneously present, a storage element must be connected to differential amplifiers 29 and 30, for which purpose the inertia of meters 31 and 32, respectively, or a capacitor may be utilized. The final values of the currents from sources and 11 will be a measure of how far the center of the pattern formed by the four positions 4, 5, 6 and 7 has had to be moved in order that these positions should lie symmetrically about the center of the phosphor dots and thus the final values of these currents are a measure of the initial mislanding of the electron beam at the part of the screen viewed. The operation can then be repeated for different parts of the tube screen in order to. obtain an overall landing characteristic. The whole operation is then repeated for the other electron guns of the tube, replacing the filter 33 of multiplier 23 by elements corresponding to the relevant color.

In practice switches 15, 18 and 24 will be formed as electronic switches so that the master contact of the switches is periodically connected to the four positions 19, 20, 21 and 22. It may be ensured that phosphor dot l is scanned once per complete picture by the electric beam originating from the active electron gun, that is to say, the four positions of the switch are obtained after a period of one twenty-fifth of 1 second. The corresponding switching frequency is therefore 25/4 6% Hz at which the switching signal must be synchronized with the field deflection signal. Since the mislanding which occurs before phosphor dot 1 will not generally differ very much from the mislanding which occurs before the phosphor dot luminescing with the same color and which is located right below phosphor dot l, the frequency may also be chosen to be 12% Hz, which facilitates the realisation of the storage element in the amplifiers 29 and 30.

The waveform of the currents flowing through coils 8 and 9 may be simplified if two square-wave forms are chosen instead of the step wave forms in FIGS. 2a and b in which one is at a maximum during the period when the other is at a minimum. In this manner only two instead of four positions of the spot are observed. Nevertheless the DC-component may be adjusted in such a manner that these two positions, one of which is, for example, position 2 in FIG. 1, are symmetrical relative to the center of phosphor dot l. The frequency of the two square-wave voltages may be 12% Hz which is one position per complete picture, or higher, namely 25 Hz as stated. It is alternatively possible to use two square-wave forms for the currents flowing through coils 8 and 9, which waveforms are not phase-shifted relative to each other, provided, however, that the measurements in the X-direction and that in the Y- direction are not performed simultaneously. If this were the case the output voltages of differential amplifiers 29 and 30 would also be zero when, for example, the observed spots would be position 2 and one position symmetrical to position 2 relative to the vertical line going through the center of phosphor dot 1.

Now that a method is known to measure accurately the landing characteristic during the manufacture of a picture tube, it is possible to use a similar method for adjusting the color purity in finished tubes, that is to say, to correct mislandings. The described apparatus cannot, however, be used without further difficulty for adjusting the color purity of the picture tube because the provision of coils 8 and 9 on the neck thereof disturbs the action of the normal purity adjusting members.

For this adjustment the coils which generate fields in the X and Y directions can be placed on the front ofthe screen of the display tube. As is shown in FIG. 4, coils 8 and 9 may be split up, for example, in two halves 8', 8" and 9', 9" which are mutually connected in series or in parallel and which are wound, for example, on a square core 35 of ferromagnetic material. In FIG. 4 the coil halves are connected in parallel. The assembly is placed against the center of the screen, whilst coil halves 8 and 8" are arranged vertically and coil halves 9' and 9" are arranged horizontally. At a given instant a current flows through coil halves 8 and 8" the direction of which current in cooperation with the windings of the coil halves 8' and 8" being such that North poles N are produced on the upper side of FIG. 4 and South poles S are produced on the lower side for both coil halves. The magnetic fluxes which are thereby induced in the core 35 eliminate each other so that the lines of force of the magnetic field generated by the coil halves 8' and 8" in the center are substantially vertically directed which causes a horizontal displacement of the electron beams, that is to say, in the X-direction. Similarly coil halves 9 and 9" cause a displacement in the Y-direction of the electron beams. A step wave current or a square wave current flows through the coil halves 8, 8" and 9', 9", however, without DC components so that sources 10 and 11 may be omitted. The purity adjusting members are now adjusted in such a manner that the spots are symmetrically arranged relative to the center of the observed phosphor dot, that is to say, in such a manner that the output voltages of differential amplifiers of 29 and 30 are zero. This adjustment actually fulfils the same function as the adjustment of the currents from sources 10 and 11 when measuring the landing characteristic. In fact, direct current fields are generated and adjusted by means of the purity adjusting members.

As described above, the square-wave current flowing through the coils may have the frequency of 12% Hz and it must be synchronized with the field deflection signal. It may be noted that different periodical current waveforms, for example, the sine waveform are alternatively suitable provided that it is synchronized with the field deflection signal, for the part of the screen which is exposed by photomultiplier 23 is small relative to the vertical size of the screen so that the said current waveform has a substantially constant value.

It is to be noted that both the measurement of the landing characteristic and the adjustment of the color purity may be performed with the aid of a microscope instead of with the described photomultiplier and the differential amplifiers. Apart from the previously mentioned drawbacks, it must, however, also be taken into account that the observed spots will flicker if the signal frequency is as low as 6%. or IZVz Hz. The more complicated apparatus of FIG. 3 is therefore to be preferred.

It has already been mentioned that the invention may alternatively be used in tubes in which the electron beam is thicker than the phosphor dot. FIG. 5 shows a possible configuration which may occur and in which l'dcnotes a phosphor dot while the spot of the electron beam may occupy the positions 4', 5', 6 and 7 in case of correct adjustment.

It will be obvious that the landing characteristic may be measured or corrected in only one direction so that one of the coils, for example, coil 9 and its associated circuitry can be omitted.

What is claimed is:

l. A method of adjusting the electron beam landing point of color display tube having at least two electron beams and groups luminescent dots of differing colors on a screen, said method comprising scanning one of said beams across at least a portion of said screen, periodically displacing the beam landing point between two points about at least one of said dots having a distance therebetween of the same order as the larger of the dot diameter and the electron beam spot diameter, whereby said two points defining a line extending in a first direction, displacing said beam in said first direction by generating a steady field, detecting the light output only from dots having the same color as said one dot, and causing the light emitted by said dot when said beam is at said two points to be substantially equal by adjusting the magnitude of said steady field.

2. A method as claimed in claim 1 further comprising periodically and steadily displacing said point in a second direction.

3. A method as claimed in claim 2 wherein said second direction is substantially perpendicular to said first direction.

4. A method as claimed in claim 1 further comprising determining the value of the initial error in said landing point by measuring the value of said steady field.

5. A method as claimed in claim 1 further comprising adjusting the color purity by varying said fields.

6. A method as claimed in claim 1 wherein said first recited displacing step comprises generating a periodic magnetic field and said steady field comprises a magnetic field.

7. A circuit for adjusting the electron beam landing point of color display tube having at least two electron beams and groups of luminescent dots of differing color on a screen, said circuit comprising means for scanning one of said beams across at least a portion of said screen, magnetic field generating means for periodically displacing the beam landing point between two points about at least one of said dots and having a distance therebetween of the same order as the larger of the dot diameter and the electron beam spot diameter, whereby said two points define a line extending in a first direction, magnetic field generating means for displacing said beam in said first direction, means for detecting the light output only from dots having the same color as said one dot, and means for substantially equalizing the light emitted by said dot when said beam is at said two points including means for adjusting the magnitude of said steady field.

8. A circuit as claimed in claim 7 wherein each of said magnetic field generating means comprises at least one coil disposed adjacent said tube, direct and alternating current sources coupled to said coil, switching means coupled between said alternating sources and said coil; said detecting means comprising a photomultiplier, a switching means coupled to said photo-multiplier and synchronized with both of said eneratin means swltc mg means, first and second ifferentia amplifiers each having two input means coupled to said detecting means switching means, said amplifiers receiving said light corresponding to the deflection caused by each of said generating means respectively, and two storage means coupled to said amplifiers respectively.

9. A circuit as claimed in claim 8 wherein each of said switching means comprise electronic switches con trolled by a switching signal having a frequency equal to or an even multiple of one fourth of the field deflection frequency and synchronized therewith.

10. A circuit as claimed in claim 8 wherein said coils are disposed adjacent the neck of said display tube.

11. A circuit as claimed in claim 8 wherein said coils are disposed adjacent said tube screen for adjusting the color purity. 

1. A method of adjusting the electron beam landing point of color display tube having at least two electron beams and groups luminescent dots of differing colors on a screen, said method comprising scanning one of said beams across at least a portion of said screen, periodically displacing the beam landing point between two points about at least one of said dots having a distance therebetween of the same order as the larger of the dot diameter and the electron beam spot diameter, whereby said two points defining a line extending in a first direction, displacing said beam in said first direction by generating a steady field, detecting the light output only from dots having the same color as said one dot, and causing the light emitted by said dot when said beam is at said two points to be substantially equal by adjusting the magnitude of said steady field.
 2. A method as claimed in claim 1 further comprising periodically and steadily displacing said point in a second direction.
 3. A method as claimed in claim 2 wherein said second direction is substantially perpendicular to said first direction.
 4. A method as claimed in claim 1 further comprising determining the value of the initial error in said landing point by measuring the value of said steady field.
 5. A method as claimed in claim 1 further comprising adjusting the color purity by varying said fields.
 6. A method as claimed in claim 1 wherein said first recited displacing step comprises geneRating a periodic magnetic field and said steady field comprises a magnetic field.
 7. A circuit for adjusting the electron beam landing point of color display tube having at least two electron beams and groups of luminescent dots of differing color on a screen, said circuit comprising means for scanning one of said beams across at least a portion of said screen, magnetic field generating means for periodically displacing the beam landing point between two points about at least one of said dots and having a distance therebetween of the same order as the larger of the dot diameter and the electron beam spot diameter, whereby said two points define a line extending in a first direction, magnetic field generating means for displacing said beam in said first direction, means for detecting the light output only from dots having the same color as said one dot, and means for substantially equalizing the light emitted by said dot when said beam is at said two points including means for adjusting the magnitude of said steady field.
 8. A circuit as claimed in claim 7 wherein each of said magnetic field generating means comprises at least one coil disposed adjacent said tube, direct and alternating current sources coupled to said coil, switching means coupled between said alternating sources and said coil; said detecting means comprising a photomultiplier, a switching means coupled to said photo-multiplier and synchronized with both of said generating means switching means, first and second differential amplifiers each having two input means coupled to said detecting means switching means, said amplifiers receiving said light corresponding to the deflection caused by each of said generating means respectively, and two storage means coupled to said amplifiers respectively.
 9. A circuit as claimed in claim 8 wherein each of said switching means comprise electronic switches controlled by a switching signal having a frequency equal to or an even multiple of one fourth of the field deflection frequency and synchronized therewith.
 10. A circuit as claimed in claim 8 wherein said coils are disposed adjacent the neck of said display tube.
 11. A circuit as claimed in claim 8 wherein said coils are disposed adjacent said tube screen for adjusting the color purity. 